The present invention relates to the field of extrusion-blow molding machines for articles made of thermoplastic material, particularly for the production of containers such as bottles, flasks or water bottles.
Such a machine essentially comprises an extrusion head that produces at least one tubular plastic parison, at least one molding unit comprising a mold in two parts, the two parts of the mold being positioned so as to close over the parison that is still soft from the extrusion head, and a blow-forming station that is provided with means for injecting air under pressure inside the parison enclosed in the mold. The air under pressure allows the parison to be deformed so that it fits the form of the mold cavity.
The machine can also comprise a degating station that allows the excess material to be cut off.
More particularly, the invention relates to a molding unit for such a machine. This molding unit comprises a mold in two parts in which each mold-half is borne by a movable support. The two movable supports are moved transversely in opposite directions with respect to a frame that holds them. In an open position, the two mold-halves are transversely separated from each other to allow a blank of the article, that is, a parison, to be inserted in a cavity delimited between the two mold-halves. In a closed position, the two mold-halves are pressed against each other by their front faces, and the supports are connected to each other by locking means.
The two supports can be movable in transverse displacement, but they can also be moved in rotation, for example around a common axis parallel to the plane of the joint of the two mold-halves.
Of course, the molding unit has a mechanism for simultaneously moving both mold supports. This mechanism is adapted to the relative path of the two supports.
In extrusion-blow molding machines, it is often provided that the molding unit comprises, between at least one of the mold-halves and the associated support, fluid pressure compensating means that push this mold-half transversely toward the other mold-half. These compensating means make it possible to prevent the two mold-halves from separating from each other under the effect of the blow pressure. They also enable the front faces of the two mold-halves to be securely closed against each other, in spite of the presence of the excess material caught between the two mold-halves at the time of closure.
An extrusion blow-molding machine incorporating these characteristics is described, for example, in the document U.S. Pat. No. 5,730,927.
The compensating means that make it possible to securely and effectively close the two mold-halves against each other exert large forces on the support that bears them. These forces increase with the size of the article to blow-mold, or in the case of a mold with multiple cavities, with the number of cavities of the mold. In effect, the forces exerted by the compensating means on the support are proportional to the blow-molding pressure, as well as to the blown surface area, in the transverse direction, of the cavity or cavities of the mold. They also depend on the amount of material caught between the two molds.
Of course, locking means are provided to resist these forces that tend to separate the two supports from each other.
However, the locking means can not prevent the support that bears the compensating means from tending to become deformed under the action of these forces.
Such being the case, this deformation, even if not very large, is then transferred to the mechanism that controls the movements of the supports. This mechanism must therefore be able to absorb without damage the forces resulting from such deformation, which generally leads to making the mechanism larger than what would be exactly needed to provide the opening-closing function of the mold.
Another solution consists of making the mold supports particularly rigid. In this case, however, the supports are then heavy and bulky, which, in addition to a higher production cost for these supports, also requires that a stronger control mechanism be provided to cause their movements. The weight of the mold supports is particularly detrimental in the case of high speed machines, in which there is a need to reduce the opening and closing time of the molds as much as possible, and thus to increase the speed of these movements, in spite of the Inertia caused by the weight of the supports.
A purpose of the invention, therefore, is to propose a new design of the molding unit that allows lighter mold supports to be used, without, however, requiring oversized means to control their movements.
To that end, the invention proposes a molding unit of the type described above, characterized in that the mechanism for moving the supports is linked at least with said associated support by elastically deformable means capable of compensating the deformation of the support produced by the forces exerted by the compensating means.
According to other characteristics of the invention:
the plate can pivot with respect to the support;
the movement mechanism acts on a plate that is slideably mounted on the support, and spring forming means are interposed transversally between the plate and the support;
the spring forming means comprise blocks of elastomer material;
the mechanism for moving the two supports comprises a drive screw that is provided with two threaded sections, the direction of turn of the threads of the two sections being opposite and each threaded section cooperating with a nut linked to one of the supports, and elastically deformable means are interposed between the nut and the support to which it is linked;
the nuts are ball-race nuts; and
the molding unit comprises compensating means only between one of the mold-halves and the associated support, and the elastically deformable means are interposed only between said associated support and the movement mechanism of the supports.
The invention also proposes an extrusion-blow molding machine, characterized in that it comprises at least one molding unit incorporating any one of the preceding characteristics.